Fernandez played the back nine first at TwinEagles Golf Club and found trouble out of the gate with a bogey at the 10th. He responded with a birdie at the par-3 12th and added a birdie at the 17th to make the turn at minus-1.
The 56-year-old caught fire on the front side and knocked an 8-iron to 10 feet for a birdie at the first. At the par-5 third, Fernandez two-putted for the first of three consecutive birdies to reach 5-under.
Fernandez held a share of the lead until the par-4 seventh. He dropped a 9-iron within two feet and converted for birdie to take the lead.
'I struck the ball well,' said Fernandez, a three-time winner on the Champions Tour. 'I took advantage of the iron shots I hit close to the hole.
'I didn't practice much during the off-season. Only played the Argentinean Open and the Argentinean PGA. I built up my golf appetite and when this year started, I really wanted to play.'
Thorpe bogeyed the par-4 fourth for his only mistake of the day. At the ninth, he hit a pitching wedge to five feet to begin a stretch of five straight birdies. Thorpe then got up and down for a birdie at the 17th and a round of 67.
'I drove the ball better today that I have in a while,' said Thorpe. 'This is a good course for me. If I play my game I'll be ok.'
Dave Barr, who became the first Canadian to win on the Champions Tour with his victory at the Royal Caribbean Golf Classic last week, had six birdies and two bogeys for a round of 68. Barr finished alongside Tom Watson, Bobby Walzel, Gil Morgan, Jim Dent, Bruce Summerhays, Gary Koch, Tom Jenkins, Isao Aoki, Terry Dill and David Eger in a tie for fifth.
Defending champion Hale Irwin struggled on the front nine with one birdie and two bogeys. The all-time winner on the Champions Tour found his groove with a birdie at the par-4 11th and added back-to-back birdies starting at the par-5 13th.
Irwin eagled the par-5 17th but found trouble with a bogey at the last to finish at 3-under-par 69. Irwin was joined by Wayne Levi and Mark Pfeil in a tie for 16th.
Des Smyth, the medalist at the Champions Tour's 2002 National Qualifying Tournament, made his debut on the circuit with a 2-under 70. The Irishman, who is the oldest player to ever win an event on the European Tour, is part of a group that features Tom Kite and Ben Crenshaw.